I haven't made it yet, but this is how you do it:
Get a sheet of #7 craft canvas & 2 report covers (you will only use the spines).
Either silicone the spines to the sides of the tank and slip in the craft canvas when it is dry, or just slip the canvas between the two spines and slide into the tank making sure that it is a snug fit. If you choose to do it this way you will have to make sure that you make the fit really snug, and push the gravel up against it (you can also use the filter intake or heater, not the hot part, as an extra support measure).
I am going to go with the second method because I don't want to have to drain the tank. Someone gave me a picture of theirs and it looks good, the fish can't see through it, and it's cheap!